91热爆

On the open plateau in the middle of a frozen volcano in Yellowstone is an animal that has lived here since the last ice age. Bison are built for the worst of the Yellowstone winter. Their thick coat is so well insulated they only need a tiny bit of energy to keep warm. So they slow their metabolism right down and concentrate on feeding. With their massive neck muscles, they sweep their heads down through the snow to get at the grass below. But the winter grass has long since put its summer goodness down into its roots and now has about the same nutritional value as cardboard. The bison will need to do all they can to save energy if they are to ward off starvation until spring returns.